The Effect of the Healthy Menstrual Cycle on the Gamma Oscillations Measured with Magnetoencephalography (MEG)

Proper balance between excitation and inhibition (E-I balance) is important for brain functioning and its disturbances are associated with many neurological and psychiatric conditions. In females, cyclic fluctuations of neural steroids, such as progesterone, during menstrual cycle (MC) are associated with changes of GABA receptors affinity to inhibitory neurotransmitter GABA, which in some cases may lead to development of premenstrual syndrome or of more severe MC-linked disorders. To find neurophysiological correlates of the changes in E-I balance regulation during MC we recorded MEG in 22 normally cycling women during the follicular and luteal phases. Participants were presented with high-contrast moving visual gratings that are known to induce strong gamma (30-90 Hz) synchronization. We analysed parameters of gamma oscillations in the primary visual cortex during stimulation intervals relative to inter-stimulus intervals. We also measured gamma response (GR) attenuation with increasing visual motion velocity, which according to our previous studies reflects E-I balance. Peak frequency of gamma oscillations increased in the luteal as compared to follicular phase of the MC pointing to increased inhibition. GR attenuation (GSS) remained stable across MC, suggesting that despite phase-related changes in regulation of neural excitation and inhibition their balance remains stable during healthy MC. The changes in gamma response power and frequency correlated with the MC-linked changes in progesterone concentration, suggesting progesterone as a key regulator of MC events. Our research establishes a basis for studying of neural biomarkers of MC-linked disorders, such as premenstrual dysphoric disorder and MC associated migraine, and potentially other brain diseases related to E-I imbalance. Besides, it shows that stage of MC is an important factor that should be controlled for in neurophysiological and cognitive studies that recruit female participants.